Among the various known processes of forming glass sheets, the processes of bending by pressing exhibit the advantage of being especially effective, since they make possible the embodiment of the most difficult glazing shapes. However, they are more difficult to use in a horizontal installation i.e., in an installation in which the glass sheets are heated in a tunnel furnace that they go through conveyed flat on a bed of driving rollers. This is because the device used to transport the glass sheets between the two presses interferes during the passage and/or induces deformations leading to optical defects.
From French Patent No. 1,398,897, a bending process is known according to which the glass sheets are heated flat in a furnace that they go through on a roller conveyor which ends at the outlet of the furnace and is lengthened by an air-cushion conveyor used to place the glass sheets between the two presses of the bending unit. The air-cushion conveyor is backed by a flexible hammock. Once the glass is correctly positioned, the air-cushion conveyor is contracted so that the glass sheet then rests on the hammock, and pressing is subsequently initiated while the hammock remains in place. The drawback of this device is that the hammock is strongly stressed mechanically and must at each bending cycle first be taut and then perfectly adapt itself to the pressing form. The life of such a hammock is consequently very brief. Moreover, the hammock always possesses a certain stiffness which prohibits the most complex shapes--for example, negative local curves (i.e. the glazing shapes comprising distributed centers of curvature on the two opposite sides of the glazing).
Further, from U.S. Pat. Nos. 3,265,484 and 4,318,728, bending installations are known in which the hammock is used directly in the transfer of glass sheets and is used as a flexible conveyor. However, the adjustment of the tension of the hammock is even more difficult than before.
Finally, from European Patent No. 228,597, a bending installation is known comprising a pressing unit with an upper bending form, solid and convex, equipped with a suction device, and an additional lower bending form. The glass sheets are transferred from the furnace to the bending unit by a device consisting of a horizontally mobile vacuum suction plate. This plate picks up a sheet, takes it to the pressing unit, and leaves again in the direction of the furnace. At the end of the pressing, the glass sheet is maintained by suction against the upper suction shape during the entire period necessary for the separation of the two forms and for the introduction, under the upper form, of an annular frame the circumference of which corresponds to the shape given to the glass sheet. The glass sheet is placed by the upper vacuum suction form on the annular frame, which takes it to a unit that cools the glass sheet and tempers it.
The known installation disclosed in European Patent No. 228,597 uses a hammock as a uniform support for the glass sheets and their positioning relative to the bending tools. The hammock is stretched above the lower bending form and is released during the pressing operation. Accordingly, from this point of view, the process is analogous to that disclosed in French Patent No. 1,398,897. Furthermore, to operate, the vacuum suction plate used for the transfer from the furnace to the pressing unit must be equipped with a vacuum pump which produces the partial vacuum necessary for the suction and with electric connections for the control of the pump motor. All this equipment must be able to withstand very high temperatures, since the vacuum suction plate enters the hottest end of the furnace, which makes it necessary to choose high-cost materials and construction techniques.